Audio amplifiers and speakers for entertainment systems can take a variety of forms. In one instance, for example, artists may interact with musical instruments, such as an electric guitar or an electric bass guitar, to generate electrical audio signals that are representative of the sounds produced by the instrument. Alternatively, the audio signals may instead be generated from vocals that are processed by a microphone. In any case, the electrical signals are routed through one or more audio amplifiers for pre-amplification, power amplification, filtering, and other signal processing to enhance the tonal quality and properties of the audio signal. The processed signals are then used to drive a speaker system to reproduce the original sound generated by the musical instrument or vocals.
While solid state electronic design continues to represent a popular paradigm within which many amplifier topologies are developed, vacuum-tube (tube) amplifier design continues to remain quite popular as well, especially with many audiophiles. Some audiophiles, for example, have become enamored with the unique sound that may be reproduced and amplified using tube amplifiers.
Perhaps the correspondence between electronic distortion and the resultant musical tone coloration produced by a tube amplifier may provide enlightenment as to the preference for tube amplification. The musical tone coloration, for example, of an audio signal may be determined by the strength of the first few harmonics of the audio signal's spectral composition. In particular, each of the lower harmonics may produce their own characteristic effect when dominant, or conversely, may modify the effect of another dominant harmonic when prominent.
Musically, the lower harmonics may be divided into two tonal groups: 1) the lower odd harmonics, e.g., the third and fifth harmonics, that produce a “stopped” or “covered” sound; and 2) the lower even harmonics, e.g., the second, fourth, and sixth harmonics, that produce a “choral” or “singing” sound. The second and third harmonics tend to be the most significant of all the lower harmonics with regard to the electronic distortion produced and, therefore, tend to be the most significant when determining the resultant musical tone coloration. Thus, the basic cause of the difference between the sound produced by tube amplification versus solid state amplification may be linked to the difference in harmonic weighting produced by the tube amplifier as compared to the transistor amplifier when each is operated in a saturated, or overloaded, state.
Transistor amplifiers, for example, exhibit a strong component of third harmonic distortion when saturated, which produces a “covered” sound having a restricted quality. Tube amplifiers, on the other hand, produce a large spectrum of harmonics when overloaded, especially the second, third, fourth, and fifth spectral components, which provides a full-bodied “brassy” quality to the sound produced.
The manner in which tube amplifiers are biased also contributes significantly to the sound quality that may be produced by the tube amplifier because the bias current, which is directly modulated by the control grid (grid) voltage, directly affects musical tone coloration. For example, an increasing magnitude of quiescent current, or bias current, conducted by the tube amplifier causes the output plate resistance and the input grid resistance of the tube amplifier to decrease. Since the harmonic content of the output signal generated by the tube amplifier is generally inversely proportional to plate resistance, an increase in bias current conducted by the tube amplifier produces a harmonically richer output signal. Accordingly, many audiophiles cannot resist the temptation to “hot bias” the tube amplifier, i.e., increase the bias current, so as to produce a richer sound.
Once an acceptable bias setting has been achieved, there exist many factors that may operate to change the bias setting. For example, individual tube characteristics, tube-to-tube variations in manufacturing, operational voltage magnitude, external component variations, etc., may affect the bias setting. In addition, the bias setting itself may have an adverse effect on tube life, whereby an increased bias current may decrease the life span of a vacuum tube contained within the tube amplifier. Thus, a user of a tube-based amplifier, such as an electric guitar amplifier, may have many occasions throughout the life span of the electric guitar amplifier to re-bias one or more tubes within the electric guitar amplifier.
Since acceptable operation of an electric guitar amplifier may be obtained over a relatively wide variation in bias settings of its constituent tube amplifiers, however, questions arise as to the frequency at which the bias settings should be adjusted. The ease by which the bias setting is adjusted also affects the frequency at which the bias settings should be adjusted, since most users do not possess the equipment or the expertise that is necessary to safely and correctly adjust the bias setting.
A need exists, therefore, to improve the ease and accuracy of the bias setting process so that optimum bias settings may be achieved to maintain optimal operation across a multitude of process and variation corners associated with vacuum tube operation.